*4.5. Water Vapor Permeability*

The procedure followed by Erdem et al. [47] was used to measure the water vapor permeability (WVP) of the films. The relative humidity (RH) of the apparatus was adjusted to 100% and 0% by using water and silica gel, respectively. Hydrogel films were sealed firmly over the glass test cups (5cm of internal diameter and 3 cm depth) containing silica gel. The cups were weighed at specific intervals to calculate the weight-gain within a day. Equation (4) was used to calculate the WVP of the films and represented in g mm/(m<sup>2</sup> )(d)(kPa).

$$\text{WVP} = \frac{\Delta \mathbf{m}}{\Delta \mathbf{t} \times \Delta \mathbf{P} \times \mathbf{A}} \times \mathbf{d} \tag{4}$$

In Equation (4), the ∆m/∆t is the weight of moisture-gain per unit of time. A is the film area in m<sup>2</sup> ; ∆P is the water-vapor pressure difference between the two sides of the film in kPa; d is the film thickness in mm.
